Thai family words are not a list you can look up once and file away. Each one carries a piece of information English leaves out, and leaves out a piece English insists on, so the translation you reach for is wrong before you have finished saying it. You already use four of these words every day without meaning to: พี่, น้อง, ลุง and ป้า go to vendors, drivers and neighbours, and that is the subject of how to address people. This page is the tree those four words were cut from, and once you have the tree the street usage stops looking like a set of arbitrary politeness rules.
Start with the household. พ่อ (phâw) is father and แม่ (mâe) is mother, and together พ่อแม่ (phâw-mâe) is the ordinary word for parents. Both are what you call your own parents to their face, and in speech there is nothing more familiar to switch down into. There is something more formal to switch up into, and you will meet it on paper rather than in conversation: บิดา (bì-daa) and มารดา (maan-daa) are the written words for father and mother, and they are what you find printed on a hospital admission form, a school form or a court document, the same way สามี and ภรรยา are. In Chinese-Thai families you will also hear ป๊า (bpáa) and ม้า (máa) in place of พ่อ and แม่. ลูก (lûuk) is a child in the sense of somebody's offspring, at any age, which is not the same thing as เด็ก (dèk), a child by age. A man of fifty is still his mother's ลูก and is not a เด็ก. ลูกชาย (lûuk-chaai) is a son and ลูกสาว (lûuk-sǎao) a daughter, and the ชาย and สาว on the end are the pattern you will meet again and again: the base word carries the relationship, and the sex is bolted on afterwards only when it is needed. ลูก is also used straight to a child as you, the way an English-speaking parent uses love or son, and a parent talking to a small child will say พ่อ or แม่ where you expect I.
For a married couple the register matters more than the vocabulary. สามี (sǎa-mii) is husband and ภรรยา (phan-rá-yaa) is wife, and those two are the words for forms, hospital counters, police stations and immigration windows, where แฟน (faen) is not an acceptable answer. In ordinary conversation แฟน does the work for all of it, and love and affection in Thai has the rest of that vocabulary. Then there is the older native pair, ผัว (phǔa) and เมีย (mia). They are not obscene and they are not recent slang; they are simply the blunt words, and couples do use them of themselves, more freely in relaxed company than in a room with anyone senior in it. The advice on the dating and family phrases is the right advice for you: understand them, and do not say them. Coming from a foreigner about somebody else's wife they land badly, and there is no version of that sentence where the formal word would have cost you anything.
Now the thing that makes Thai kinship genuinely different, and the reason a learner freezes mid-sentence. Thai ranks siblings by age. English ranks them by sex. Your first and default word is พี่ (phîi), an older sibling, or น้อง (náwng), a younger one, and neither of them tells anybody whether the person is a man or a woman. If the sex matters, and often it does not, you add it: พี่ชาย (phîi-chaai) older brother, พี่สาว (phîi-sǎao) older sister, น้องชาย (náwng-chaai) younger brother, น้องสาว (náwng-sǎao) younger sister. English does the exact opposite. Brother and sister are compulsory about sex and silent about age, so an English speaker who wants to say brother in Thai has to supply a fact he has never had to think about, and that is the moment the sentence stalls. There is no neutral word for sibling to escape into. The nearest thing is the pair พี่น้อง (phîi-náwng), which means siblings collectively and is what the standard question uses.
Sort this out before you need it. Decide which of your own brothers and sisters are older than you, and learn the words attached to the actual people, because มีพี่น้องกี่คน (mii phîi-náwng gìi khon), how many brothers and sisters do you have, is the second question you will be asked after your name. Thai speakers differ over whether they count themselves in the total, so the answer that avoids the ambiguity names them rather than giving a bare number: ผมมีพี่สาวหนึ่งคน น้องชายสองคนครับ. Birth order is asked about directly too, with เป็นลูกคนที่เท่าไหร่ (bpen lûuk khon thîi thâo-rài), which number child are you, and answered with ลูกคนโต (lûuk khon dtoo) eldest, ลูกคนกลาง (lûuk khon glaang) middle, ลูกคนเล็ก (lûuk khon lék) youngest, or ลูกคนเดียว (lûuk khon diao) only child. The question is not idle: the eldest is expected to support the parents, so your position in the order tells a Thai listener something about your life that your job does not.
Cousins fall into the same ranking. ลูกพี่ลูกน้อง (lûuk-phîi-lûuk-náwng), literally the children of older and younger siblings, is the word for cousin, and it marks neither the side of the family nor the sex. In practice it is a describing word rather than an address: you call a cousin พี่ or น้อง exactly as you would a brother or a sister. That is one reason a Thai friend speaking English will call a cousin his sister, and why you should ask เป็นพี่น้องกันหรือลูกพี่ลูกน้องกันครับ before you draw any family tree from an English sentence. Note the question uses พี่น้อง rather than a sex-and-age word, because you do not yet know which of them is older and that is exactly what you are not entitled to assume.
The generation above your parents runs on two axes, and the axes swap over halfway. For anyone older than your own parent, the side does not matter and the sex does: ลุง (lung) is the elder brother of either your mother or your father, and ป้า (bpâa) the elder sister of either. For anyone younger than your parent it flips: น้า (náa) is your mother's younger sibling and อา (aa) your father's younger sibling, and the sex is not marked at all. So the axes really do swap. Above your parent, Thai marks the sex and ignores the side; below your parent it marks the side and ignores the sex. What it never lets go of, in either direction, is whether the person is older or younger than the parent they belong to. Where you do need the sex of a น้า, add it as usual, น้าชาย (náa-chaai) and น้าสาว (náa-sǎao); อา is more often left unmarked, and Thais who need the distinction say อาผู้ชาย or อาผู้หญิง. The practical payoff is that when someone at a family meal is introduced to you as น้า, you have been told without a word of explanation that she is on your partner's mother's side and younger than her. Outside the family the same words work as forms of address on apparent age alone, with the cautions that how to address people sets out: ป้า is riskier than ลุง, and อา stays largely inside families and Chinese-Thai households.
Grandparents mark the side, and in a Thai family they mark it every time. On your father's side, ปู่ (bpùu) is his father and ย่า (yâa) his mother. On your mother's side, ตา (dtaa) is her father and ยาย (yaai) her mother. Four words, four positions, and the pair you use is itself a statement about which of your parents you are tracing through, which is why nobody has to ask whose grandmother is meant. ตา is also the ordinary word for eye, which should not make you doubt the kinship sense when you hear it inside a family. Above them, ทวด (thûat) is a great-grandparent on either side, and you will need it about as often as you need the English word. Going downward Thai economises where English does not: หลาน (lǎan) is a grandchild, a niece and a nephew all in one word, and it says nothing about which generation or which sex. หลานชาย (lǎan-chaai) and หลานสาว (lǎan-sǎao) add the sex and still leave the generation open, so if you need to know whether the child in the photograph is a grandchild or a sibling's child you have to ask. เหลน (lěen) is a great-grandchild. One set sits outside this. In Chinese-Thai families you will hear อาม่า (aa-mâa) for a grandmother and อากง (aa-gong) for a grandfather, and neither of them marks a side; the address lesson has them alongside เจ๊ and เฮีย for the market.
The in-law words exist because Thai needs to say which marriage a person came in through, and they are almost entirely words for describing somebody rather than for addressing them. Your wife's parents are พ่อตา (phâw-dtaa) and แม่ยาย (mâe-yaai); read the compounds and they explain themselves, since they are the father and mother who are ตา and ยาย to your children. Your husband's parents are พ่อสามี (phâw-sǎa-mii) and แม่สามี (mâe-sǎa-mii), built instead on the word for husband. You will hear the blunt pair as well: พ่อผัว (phâw-phǔa) and แม่ผัว (mâe-phǔa), built on ผัว and carrying the same bluntness the word does on its own, and แม่ผัว in particular is the half you meet in stories about a difficult mother-in-law, fixed in place opposite ลูกสะใภ้. Recognise them; use พ่อสามี and แม่สามี. The asymmetry is in the vocabulary, not in your behaviour: to their faces both sets are คุณพ่อ (khun phâw) and คุณแม่ (khun mâe) from the first minute, and they stay that way until the family itself moves you down to plain พ่อ and แม่, which happens when you are counted as family rather than as a visitor and is not something you initiate. Saying แม่ยาย to the woman herself is not a form of address at all. For the people who married into your own family, there are two suffixes and one rule. สะใภ้ (sà-phái) marks a woman who married into the family and เขย (khǒei) a man, and the word each attaches to is the blood relative through whom the link runs. So ลูกสะใภ้ (lûuk-sà-phái) is a son's wife, ลูกเขย (lûuk-khǒei) a daughter's husband, พี่สะใภ้ (phîi-sà-phái) an older brother's wife, น้องเขย (náwng-khǒei) a younger sister's husband, and so on through the tree. If you are a foreign man married to a Thai woman, ลูกเขย is what you are, and you are likely to hear the word used about you in the third person before anybody explains it.
Two umbrella words are left. ครอบครัว (khrâwp-khrua) is family, and in everyday use it usually means the household, the people who live in the house and depend on each other; whether it stretches to grandparents, aunts and cousins depends on who is speaking, so do not read a fixed boundary into it. ญาติ (yâat) is a relative, and it stretches much further than the English word. It takes in blood relatives and people married in without distinguishing them, it keeps counting through grandparents and their siblings well past the point where an English speaker would stop, and you will be introduced to people as ญาติ whose exact position nobody in the room may be able to draw on paper. The set phrase is ญาติพี่น้อง (yâat phîi-náwng), the whole connection taken together. Asking เป็นญาติกันยังไงครับ, how exactly are you related, is a normal question and often gets a long answer. Two compounds are worth having: ญาติสนิท (yâat sà-nìt), a close relative, and ญาติผู้ใหญ่ (yâat phûu-yài), the senior relatives, who are the people whose view is asked when a family decides something. On a hospital admission form ญาติ is the word in the box that wants a contact number, and it wants a relative, which is a problem of its own if you live here alone; the hospital and insurance phrases cover what to say at that counter.
What happens when you are taken to meet the family is less mysterious than it is usually made to sound, and most of it is one instruction: greet the most senior person present first and work down. That means the grandparents before the parents, the parents before your partner's older siblings, and your partner last of all, whatever the two of you would do at home. A wai with สวัสดีครับ or สวัสดีค่ะ is the whole greeting, and the wai lesson has the height of the hands: for your partner's parents it is thumbs to the nose, not chest level, and you wai first rather than waiting to be waied at. The second instruction is that you address every one of them with the kinship word rather than a name. คุณพ่อ and คุณแม่ for the parents, ป้า, ลุง, น้า and อา for the generation above, พี่ for anyone older than you, and the word is doing the job the name would do at home. For yourself the rule is shorter. A man stays on ผม (phǒm) with the parents and every elder in the room, all evening, however warm it gets. A younger woman uses หนู (nǔu), which is what a girl or a young woman says for I to anyone clearly older and which the pronouns lesson sets out; หนูชื่อ… is how she introduces herself at the door, and a woman who is past that uses ฉัน (chǎn) instead. Neither of you switches to a nickname with the parents, whatever you do with the cousins. You will usually be told which word to use as you go, in the form นี่ป้านะ, this is your aunt, and that sentence is the introduction, so you may go a long time without ever learning some of these people's names. When nobody tells you, ask your partner quietly, and until you have an answer use พี่ for anyone who might plausibly be older, which is the default the address lesson argues for and which is at worst a small over-politeness. Its one caveat holds here too: aimed at someone plainly much younger than you, พี่ sounds like a joke. Beyond that, expectations vary far more than any guide admits: what a family wants from you in visits, gifts, money and ceremony differs by family, by region, by generation and by what the household can afford. The dating and family phrases handle those specific conversations, sin sot included, where the figure is set by the older generation and relayed to you, often by an aunt or an uncle rather than by the parents, though the question itself may well come from the father. Do not take a rule off a forum and apply it to a family you have met twice. Ask your partner what her family does.
Two more categories, because they come up more often than the textbooks suggest. Step-relations take เลี้ยง (líang), to raise: พ่อเลี้ยง (phâw-líang) stepfather, แม่เลี้ยง (mâe-líang) stepmother, ลูกเลี้ยง (lûuk-líang) stepchild. In the north you will also hear พ่อเลี้ยง used of a well-off local businessman, which has nothing to do with anybody's family. An adopted child is ลูกบุญธรรม (lûuk bun-tham). And a child with one Thai and one foreign parent is ลูกครึ่ง (lûuk-khrêung), literally half-child, an ordinary descriptive word with no edge on it.
Finally, the reason to learn all of this even if you never meet anybody's family. These words are not confined to relatives. พี่ and น้อง run the service economy, ลุง, ป้า and น้า go to strangers in the street, all of them stand in for I as readily as for you, which the pronouns lesson covers, and the family words are welded into ordinary nouns you meet in a market in your first week: แม่ค้า (mâe-kháa) is a female vendor and พ่อค้า (phâw-kháa) a male trader, แม่บ้าน (mâe-bâan) is a housekeeper or a housewife depending on the sentence, ลูกค้า (lûuk-kháa) is a customer, and ลูกน้อง (lûuk-náwng) is a subordinate at work. Thai builds relationships out of the family words the way English builds them out of job titles, so the tree on this page is not background reading for a wedding you may never attend. It is the shape of a great many conversations you will have here.